1. Field of the Invention
The present invention relates to a magnetic recording medium used in recording of a information. More particularly it is used in an external memory of computer, video or audio recording apparatus, game machine or other memory device, or integrated multi-media.
2. Description of the Prior Art
Recently, read-only recording media are widely spread for audio, video and data files, such as CD, LD, and CD-ROM. However, along with the distribution of optical recording media, lately, as versatile data are handled, there is a growing demand for higher density.
As one of the solutions, in the rewritable magneto-optical recording media, it is proposed to read a tiny written mark smaller than a laser spot diameter by the following method. That is, in the constitution having a separate recording layer and a reading layer, only in a limited narrow region in part of an irradiation region of an optical beam for reading, the information recorded in the recording layer is read while forming a state copying into the reading layer, and this method is called super-resolution reading.
This is the method disclosed in the Japanese Laid-open Patent No. 03-93058, and basically it is composed of two perpendicular magnetized films comprising a recording layer of high coercive force and a reading layer of low coercive force, and an exchange coupling force acts between the magnetized films. The reading principle of the thus composed magneto-magneto-optical recording medium is briefly described below. At room temperature, only the magnetization of the reading layer is aligned in one direction by a powerful external magnetic field (3 Koe or more). However, in the region elevated in temperature as the optical beam for reading is irradiated, the coercive force of the reading layer drops abruptly, and the written mark of the recording layer is copied in the reading layer by the exchange coupling action. Therefore, signals are generated only from the portion heated by the irradiation of optical beam for reading, whereas no signal is generated from the portion not elevated in temperature in the region irradiated with optical beam for reading.
This method is effective means for the recording medium to be recorded by the user as in the conventional magneto-optical recording, but is not applied to the read-only magneto-optical recording medium. As the method of super-resolution reading on a read-only magneto-optical recording medium, a method of a making use of difference between the reflectance of solid-phase state and reflectance in liquid-phase state is known (Yasuda et al. International Symposium Optical Memory and Optical Data Storage '93 Th3.2, 1993). In this method, GeSbTe is disposed on a substrate in which information is recorded in pit form, and a reflectance layer is further laminated. The principle of reading of the magneto-optical recording medium in this constitution is briefly described below. When reading by irradiating an optical beam from the substrate side, at room temperature, GeSbTe is in a solid-phase state, and the light transmittance of GeSbTe is large, and hence a certain reflected light is obtained. However, when the temperature is elevated by the optical beam for reading, GeSbTe is set in a liquid-phase state, and the light transmittance of GeSbTe decreases significantly, and the reflected light becomes extremely small.
Therefore, the read signal is limited to a cold portion only of the region irradiated with an optical beam for reading, and super-resolution reading is enabled in the read-only magneto-optical recording medium.
In such constitution, however, there are the following defects. The first problem is that an extremely large power is required in spite of read-only use because the reading temperature must be more than several hundred degrees in order to read while heating always to the liquid-phase state of GeSbTe. It means not only that a large output laser is needed, but also that substrate deterioration due to repeated reading or deterioration of GeSbTe film may result.
The second problem is that it is likely to pick up the crosstalk of the adjacent track to make it hard to narrow the track pitch because only the cold portion is read while masking the hot portion of the region irradiated with optical beam.
The third problem is that it is hard to use part of the recording medium for reading only and the other as a rewritable recording medium, that is, so-called partial ROM, or rewritable data file having read-only address information or control information. That is, the GeSbTe film itself is a rewritable recording medium making use of reversible changes between crystal state and amorphous state, but if the information is recorded by keeping this film always in a liquid-phase state, it is read while erasing it, and the phase-change recorded information cannot be read again.